576 
SEMIVER TERRA TES. 
appear to have been derived by a series of changes mainly correlated with the 
assumption of their characteristic radial symmetry; while the Chordates retained 
the original bilateral symmetry, at the same time undergoing certain changes, into 
the consideration of which it will be unnecessary to enter in this place. Still more 
complicated are the changes necessary to evolve Balanoglossus and its allies from 
the ancestral form. Such of our readers as are desirous of pursuing further this 
interesting subject, may be referred to the works of the observer last mentioned. 
A PYROSOMA-COLOXY. 
In life the colony assumes a horizontal position. 
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